1. Field of the Invention
This invention relates to a method of suppressing jet engine noise and an ejector system therefor.
2. Prior Art Relating to the Disclosure
A great deal of effort has been expended to develop a practical and economical method of suppressing "jet noise" caused by the shearing action of the high-velocity combustion exhaust gases acting on the surrounding ambient atmosphere on their discharge. While one can suppress jet noise by reducing the velocity of the exhaust gas steam, this also means a reduction in engine thrust unless the mass is increased to compensate for the reduction in the velocity of the exhaust gas steam. The following patents illustrate and describe various ejector systems for attenuating and reducing jet engine noise.
U.S. Pat. No. 3,710,890 describes an exhaust nozzle systyem for suppressing jet engine noise wherein the exhaust flow of the engine is ducted into the inside of a series of corrugated, fluted lobes surrounding the primary gas stream. Secondary air is entrained by ejector action between the outside of the fluted lobes and the inside of the ejector shroud. The shape and size of the lobes are designed to convey the exhaust gases from their original cylindrical pattern into a larger annular region within the shroud for mixing with large quantities of ambient air drawn by ejector action into the shroud to create greater mass flow of the exit gas stream at reduced velocity.
U.S. Pat. No. 4,175,640 describes reduction of the maximum velocity of gases at the nozzle end of a turbofan engine by creation of a pattern of vortex flow of a mixed stream of primary and fan gases.
U.S. Pat. No. 3,667,680 describes an exhaust nozzle system for turbojet afterburning engines employing a two-stage, variable area ejector nozzle to maintain an approximately Mach 1.0 engine exhaust exit velocity.
U.S. Pat. No. 4,117,671 recognizes the difficulty of maintaining thrust by increasing the length of the mixing section of an ejector in a ducted-fan, turbojet engine due to frictional drag and other drag losses and seeks to minimize these losses by a mixing assembly wherein the bulk of the primary flow is gradually redirected to diverge radially outwardly and the bulk of the fan flow gradually redirected to converge radially inwardly.
U.S. Pat. No. 4,149,375 discloses an improved lobe-type mixer for a fan jet engine wherein the sidewalls are scalloped.
U.S. Pat. No. 4,137,992 describes inserting noise-absorbing structures within the nozzle of the engine to attenuate noise.
U.S. Pat. No. 3,954,224 discloses a retractable, multi-tube exhaust element having means for increasing the supply of ambient air thereto.
U.S. Pat. No. 4,165,609 discloses a flow mixer for a turbofan engine in which axially extending and circumferentially interspersed regions of the coaxially flowing fan stream and primary flow streams are caused to penetrate each other.
U.S. Pat. No. 4,077,206 discloses a "daisy wheel" type of tubular mixer of particular design for attenuating the core noise associated with turbine exhaust gases and the aft fan noise.
U.S. Pat. No. 3,065,818 discloses a jet noise suppressor nozzle having a plurality of hollow flow diverter members closed at their upstream ends and open at their downstream ends, with means for entraining atmospheric air into and through the interior of some of the flow diverters.
U.S. Pat. No. 3,721,314 discloses a plurality of scoop-like members movable into and out of the jet exhaust of a jet engine, the scoops designed to bleed off portions of the hot gas flow and secondary flow and discharge them to the ambient atmosphere.
U.S. Pat. No. 3,695,387 discloses a shroud for a turbofan jet engine in which circumferentially spaced ducts are pivotally mounted to swing into a position to partially block the rearwardly flow of exhaust gases and divert them radially outwardly into the fan airstream to promote intermixing of the primary and fan airstreams prior to their leaving the shroud.
U.S. Pat. No. 3,737,005 discloses an ejector nozzle wherein the inner walls of the ejector nozzle are formed of acoustical and structural honeycombed paneling perforated on the mixing zone side.
U.S. Pat. No. 3,749,316 discloses an ejector nozzle wherein jet nozzles are arranged around the walls of an ejector ring to discharge energized gas to form an axial peripheral jet stream to surround and accompany the mixed gases flowing out of the ring.
U.S. Pat. No. 3,820,630 discloses an ejector nozzle noise suppressor which causes the exhaust gases to overexpand in a cavity immediately on exit from the nozzle to produce a shock wave, causing the exhausted jet to become subsonic.
Some experimental work was carried out by the applicant on an ejector assembly designed to give a Mach 1.0 entrance condition for the secondary gas stream; however, there was no apparent reason observed, from the initial tests, why the primary and secondary gas streams would mix without a relatively long mixing section or zone.
There is no recognition in any of the patents cited of the design of an ejector nozzle having a mixing and diffusing section where secondary air is pulled into the mixing section at a velocity sufficiently high so that the mixed flow attains a sonic or choked condition in the mixing section, thereby rapidly mixing the secondary and primary gas flows at the exit end of the mixing section and substantially suppressing the jet noise without reduction of the thrust of the engine.